Accurate, non-invasive blood flow measurement has long been a goal of medical researchers. Non-invasive measurement of blood flow in the limb and especially the leg where most vascular diseases occur would facilitate early detection of such diseases. Such early detection would enable diagnosis and treatment of the disease before limb amputation would be necessary. As detailed in the paper "The NMR Blood Flowmeter, Theory and History" by Joseph H. Battocletti, Richard E. Halbach, Sergio X. Salles-Cunha, and Anthony Sances, Jr., published in the July/August 1981 issue of Medical Physics, Vol. 8(4) at pages 435-443, following the discovery of the phenomenon of nuclear magnetic resonance, scientists began experimenting to determine if liquid flow could be measured non-invasively using the principles of nuclear magnetic resonance. It was soon discovered that the nuclear magnetic response signal induced in a moving, paramagnetic fluid was low when fluid velocity was low due to saturation in the detector but the NMR signal magnitude increase in response to an increase in fluid velocity as a consequence of magnetized nuclei entering the detector from upstream of the saturation region.
Later advances in the field of nuclear magnetic resonance fluid measurement included the discovery that fluid flow could be detected by magnetically "tagging" a bolus of the fluid. By physically locating a small magnetic coil, commonly referred to as a "tag" coil ahead of the NMR transmitter and receiver coils such that the tag coil, when energized with a fixed radio frequency signal produced a field orthogonal to the polarizing field, a bolus of fluid could be given a magnetic orientation different from an adjacent bolus. When this tagged bolus reached the receiver coil, a brief unidirectional pulse would be induced in the receiver coil. From a knowledge of the time that had elapsed between tagging of the bolus and detection of the tagged bolus at the coil receiver and from a knowledge of the distance between the tag coil and the receiver coil, fluid velocity could be determined. Examples of nuclear magnetic resonance fluid flowmeters for measuring fluid flow in this manner are found in U.S. Pat. Nos. 3,419,793, 3,419,795, 3,551,794 and 3,473,108.
Nuclear magnetic resonance fluid flow measuring methods have been employed to construct experimental blood flowmeters for non-invasive measurement of blood flow. The structure of a typical, present day experimental nuclear magnetic resonant blood flowmeter is described in the paper "A Nuclear Magnetic Resonance Non-invasive Leg Blood Flow-Meter" by J. H. Battocletti, R. E. Halbach, A. Sances, Jr. and F. J. Antonich published in the Conference record of the IEEE 1981 Conference "Frontiers of Engineering Health Care," Houston, Tex., Sept. 19-21, 1981 at pages 145-147. Generally, a NMR blood flowmeter includes a pair of polarizing magnets which serve to polarize the blood flow flowing in a bodily member, such as a limb which is disposed within a lumen located between the magnet poles. A receiver coil and a transmitter coil, typically arranged so that the field direction of the coils is orthogonal to each other, are carried by the lumen so as to be located between poles of the polarizing magnet. Located on the lumen upstream of the detector and transmitter coils is a tag coil which is oriented such that when energized with a fixed radio frequency signal, the tag coil generates a magnetic field orthogonal to the polarizing field so as to tag or demagnetize a bolus of blood. When this tagged bolus reaches the receiver coil, the output signal of a receiver coupled to the receiver coil will deviate. The deviation in the receiver output signal is detected by a detector. By measuring the time between deviations of the receiver output signal and by knowing the distance between the tagging coil and the detector, blood flow can then be calculated.
Further research in the field of non-invasive limb blood flow measurement using the principle of nuclear magnetic resonance has led to the development of a nuclear magnetic resonance blood flowmeter which enables ranging or focusing of the nuclear magnetic resonance response within a single plane. Thus, the limb bloodflow can be "imaged" in a single plane in a manner similar to the way in which X-rays are now imaged. An experimental nuclear magnetic resonance blood flowmeter capable of one dimensional imaging or ranging has been described in the paper "Ranging for Individual Artery Flow in the Nuclear Magnetic Resonance Flow Meter" by Richard E. Halbach, Joseph A. Battocletti, Anthony Sances, Robert Bowman, and Vsevolod Kudravcev published in the conference record of the IEEE 1981 Conference "Frontiers of Engineering in Health Care", Washington, D.C., Sept. 28-29, 1980 at pages 356-359. The experimental blood flowmeter described in the Halbach et al. paper is similar to that described previously except that it also includes a pair of scanning or ranging coil sets. The coils are each located on opposite sides of the lumen or limb receiving cavity so as to be orthogonal to the polarizing magnets of the nuclear magnetic resonance blood flowmeter. When the coils of each of the pair is energized with a current opposite in polarity to the current in the other of the scanning coils, each scanning coil generates a magnetic field opposite in polarity to the field generated by other of the pair of scanning coils. These opposing fields tend to cancel the nuclear magnetic resonance response everywhere except in a null plane where the fields of the scanning coils tend to cancel themselves. By varying the ratio of the current in the scanning coils, the null plane of the scanning coils, that is, the plane of the nuclear magnetic response can be shifted across the lumen to allow imaging of the limb in one dimension.
While the scanning type nuclear magnetic resonance blood flowmeter, such as described in the Halbach et al. paper, accomplishes one dimensional scanning or ranging of the nuclear magnetic resonance response induced in the blood, thereby enabling blood flow measurement within a single artery to the exclusion of others, one dimensional ranging or imaging-type nuclear magnetic resonance blood flowmeters do not provide any indication of arterial or venous blood flow along a particular base line. To accomplish non-invasive blood flow measurement within an artery or vein along a certain base line would require imaging in two dimensions, something that has not heretofore been accomplished by prior art nuclear magnetic resonance blood flowmeters.
In order for present day experimental nuclear magnetic resonance blood flowmeters, such as the type disclosed in the Battocletti et al. paper or the ranging type nuclear magnetic resonance blood flowmeter described in the Halbach et al. paper to accomplish accurate detection of blood flow, the magnetic field of the polarizing magnet must be uniform and remain relatively constant. To provide for coarse adjustment of the uniformity of the magnetic field generated by the polarizing magnets, mechanical shims, taking the form of rectangular steel strips, are placed on the pole faces. Relatively fine adjustment of the magnetic field is accomplished by electromagnetic coils located around the magnet poles. The electromagnetic coils are energized by a closed loop control circuit responsive to the polarizing magnet flux. In the past, Hall effect sensors have been utilized as the flux sensing element for the closed loop control circuit energizing the pole face electromagnetic coils. However, such Hall effect sensors are typically temperature sensitive so that the flux measurement provided thereby tends to vary with the ambient temperature of the Hall effect sensor. Thus, as the magnet temperature rises, the Hall effect sensor temperature rises and the magnetic flux sensed by the Hall effect sensor will thus vary, resulting in undesirable variations in the magnetic flux generated by the electromagnetic coils. This ultimately causes an undesirable variation in the polarizing magnet flux.